9 Causes of Unintentional Weight Gain

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Gaining weight when you don’t want to is so common that it is almost the rule rather than the exception. There are several potential causes for unintentional weight gain. So, if the number on your scale is rising and you genuinely don’t know why, here are nine things to look for as the possible cause.

Foods Consumed

While it’s true that the number of calories we store as fat can be reduced to a simple equation (calories consumed - calories burned = calories stored or lost), human physiology makes gaining or losing weight a bit more complicated than that. 

To our bodies, food does not merely represent calories and nutrition. Food is also information. The kind of food we eat tells our bodies something about our environment and, in response to that information, our body changes several things.

It changes the way we handle and store the calories we consume. It adjusts whether our appetites are stimulated or suppressed. It also slows or speeds up our metabolism. This means, among other things, that the type of food we eat has an impact on our weight, aside from how many calories it contains.

As an example, when we eat a lot of carbs with a high glycemic index, we boost our insulin levels. Think of insulin as fat’s jailer—it locks up fat in our fat cells, refuses to release it, and prevents us from burning it off. By avoiding high-glycemic carbs, we can reduce our insulin levels and help release that “incarcerated” fat.

Eating Habits

Eating patterns can also affect weight gain. Eating five smaller meals a day versus two or three bigger ones, for instance, has been associated with a reduced risk of becoming overweight.

Eating breakfast regularly is also thought to reduce the risk of weight gain, although there are pros and cons to this practice. Conversely, eating before bedtime has been associated with obesity (and sleep disturbances).

Keep in mind that, as we age, our bodies change how we handle calories. As a result, we tend to gain weight far more easily later in life than we did in our youth. This weight gain often occurs without any discernible change in our diet or activity level, so it is “unexplained.”

Insufficient Physical Activity

Living a sedentary lifestyle is strongly associated with weight gain. Engaging with lots of electronics, for example—such as when watching TV, gaming, or scrolling through social media—is a sedentary behavior that has been found to increase obesity risk.

Regular exercise not only burns calories directly but it also can jazz up our metabolism so we burn more calories between exercise sessions. Understand how much exercise you really need and make it a point to get it.

Sleep Deprivation

Getting less than 7 hours of sleep per night is associated with gaining weight. The reason for this, at least partially, is that sleep deprivation decreases the appetite suppressing hormone leptin and increases the appetite stimulating hormone ghrelin.

So, when we are sleep deprived, we are hungrier and we eat more. This can easily lead to weight gain if your lifestyle is one that involves repeatedly going without adequate sleep.

Smoking Cessation

While a major boon to health, quitting smoking can lead to weight gain. In fact, some experts suggest that smoking cessation programs should always include dietary and exercise components to prevent excess gains in weight.

Taking Certain Prescription Drugs

Several prescription medications have been associated with weight gain, including:

  • Several drugs used to treat psychiatric disorders such as schizophrenia, depression, and bipolar disorder
  • Steroids
  • Birth control pills
  • Some diabetes drugs
  • Some drugs used for seizure disorders

If you are taking any prescription medications, talk to your doctor about whether your medicine might be contributing to your weight gain.

Endocrine Disorders

Several disorders of the endocrine system often lead to weight gain. These include:

  • Cushing's syndrome
  • Hypothyroidism
  • Polycystic ovary syndrome (PCOS)
  • Growth hormone deficiency

Check with your doctor to see whether you should be evaluated for any of these problems.

Cardiac Disease

Heart failure is the result of several kinds of cardiac disease and is associated with weight gain because of fluid retention. People with heart failure often develop edema in the legs, but can retain as many as 20 kg (44 pounds) of fluid in the body as a whole.

Other Medical Issues

Several other medical problems can produce weight gain by causing fluid retention. For instance, kidney disorders, especially nephrotic syndrome, can lead to significant weight gain from edema.

Similarly, liver problems such as cirrhosis can produce fluid retention, especially in the abdomen (a condition called ascites). Lymphedema, fluid retention caused by blockage in the lymphatic system, can also cause weight gain. 

A Word From Verywell

The most common causes of weight gain are related to poor diet and exercise habits. If you are eating reasonably and getting a fair amount of physical activity, you should consult with your doctor about whether you might have one of these other potential causes of weight gain.

11 Sources
Verywell Fit uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Richard Fogoros, MD
Richard N. Fogoros, MD, is a retired professor of medicine and board-certified in internal medicine, clinical cardiology, and clinical electrophysiology. He co-invented 19 awarded U.S. patents relating to the detection of cardiovascular disease, and is the author of numerous scientific articles, book chapters, and books.